Visceral pain is the most common form of pain and is one of the most difficult forms of pain to treat, often with the use of opioids. Visceral pain is distinct from somatic pain and is generally described as pain that originates from the body's internal cavities or organs and has five important clinical and sensory characteristics: (1) it is not evoked from all visceral organs (eg liver, kidney, lung); (2) it is not always linked to visceral injury (e.g., cutting an intestine does not evoke pain); (3) it is diffuse; (4) it is referred to other locations; and (5) it can be referred to other autonomic and motor reflexes (e.g., nausea, lower-back muscle tension from renal colic) (Lancet, 1999, 353, 2145-48). Several theories have been proposed for the mechanisms of visceral pain. In the first theory, the viscera are innervated by separate classes of neurons, one concerned with autonomic regulation and the other with sensory phenomena such as pain. The second theory suggests a single homogenous class of sensory receptors that are active at low frequencies (normal regulatory signals) or at high frequencies of activation (induced by intense pain signals). However, studies indicate that the viscera is innervated by two classes of nociceptive sensory receptors: high threshold (mostly mechanical receptors found in heart, vein, lung, airways, oesophagus, biliary system, small intestine, colon, ureter, airways, urinary bladder and uterus; activated by noxious stimuli) and low threshold intensity coding receptors that respond to innocuous and nocuous stimuli (heart, oesophagus, colon, urinary bladder and testes). Yet another theory suggests a component of afferent fibres that are normally unresponsive to stimuli (silent nociceptors) that can become activated or sensitized during inflammation (Trends Neurosci. 1992, 15, 374-78). Once sensitized, these nociceptors now respond to innocuous stimuli that normally occur in the internal organs resulting in an enhanced barrage of convergent input to the spinal cord and subsequently triggering central mechanisms that amplify the effect of the peripheral input.
Compounds for the treatment of visceral pain would therefore be highly desirable.